Heat butter in a 12-inch heavy skillet over medium heat until foaming, then cook onions with scallion, garlic, salt, and 1/4 teaspoon pepper, stirring occasionally, until softened but not browned, about 5 minutes,______, Add 1/2 cup water and simmer gently, stirring, until shrimp are just cooked through, 2 to 3 minutes (shrimp should be saucy; add more water if necessary), Season with salt and pepper and serve immediately,